The trend to increase the speed and bandwidth of communication networks has grown also in subscriber access networks, and passive optical network systems (PONs) stipulated by Recommendations G984.3 and the like of Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) have been introduced. The PON is a system that connects an optical line terminator (OLT) connected to an upper communication network and an optical network unit (ONU) serving a plurality of subscriber terminals (PCs and telephones) by a passive optical network that includes a trunk optical fiber, an optical splitter, and a plurality of branch optical fibers. More specifically, communication is conducted as follows: Signals from the terminals (such as a PC) connected to each ONU sent as optical signals through the branch optical fibers pass an optical splitter, are optically (time division) multiplexed on the trunk optical fiber, and are sent to the OLT; and the OLT performs communication processing of the signals from the ONUs and send the signals to the upper communication network or to another ONU connected to the OLT.
In the early period, PON systems that handle signals at a low speed such as 64 kbps were introduced. Broadband PONs (BPONs) that send and receive fixed-length ATM cells at about 600 Mbps at a maximum, Ethernet PONs (EPONs) that send and receive variable-length Ethernet (registered trademark, the indication of the registered trademark will be omitted below) packets at 1 Gbps at a maximum, and gigabit-capable PONs (GPONs) that can handle signals at a faster speed of about 2.4 Gbps have been introduced. A high-speed PON that can handle signals at a rate ranging from 10 Gbps to 40 Gbps will be demanded in the future. As a means for implementing those high-speed PONs, use of time division multiplexing (TDM) as used in the current PONs is considered. The current PONs that use TDM use different wavelengths for the upstream signal (from the ONU to the OLT) and the downstream signal (from the OLT to the ONU), and communication between the OLT and the ONUs is implemented by assigning communication time slots to the ONUs. More specifically, a variable-length burst signal, which can be easily applied to a variety of signals (such as sound, image, and data), is assigned.
In the PONs described above, the ONUs are disposed in scattered subscriber's places, and the ONUs are at different distances from the OLT. The variety of lengths (transmission distances) of the optical fibers that include the trunk optical fiber and the branch optical fibers between the OLT and the ONUs cause variations in transmission delay (delay amount) between the ONUs and the OLT. Even if the ONUs send signals at different timings, the optical signals from the ONUs may collide or interfere with one another on the trunk optical fiber. Accordingly, the PONs use the ranging technique stipulated in the ITU-T Recommendation G984.3, for example, to measure the distances between the OLT and the ONUs and adjust the delay of signals output from the ONUs so that the output signals will not collide with one another. The OLT determines the bandwidth of a signal that the ONU is permitted to send, by using the dynamic bandwidth assignment (DBA) technique, in accordance with transmission requests from the ONUs, and specifies the transmission timings of the ONUs such that the optical signals from the ONUs will not collide or interfere with one another on the trunk optical fiber, taking the delay amount measured by ranging into consideration. The PONs are configured to perform communication by managing the timings of signal transmission and reception between the OLT and the ONUs in the system.
In the GPON, in order to allow the OLT to identify and process the signals sent from the ONUs and multiplexed on the trunk optical fiber, the transmission signal from each ONU begins with a guard time of up to 12 bytes provided to prevent interference; a burst overhead that includes a preamble used to determine a signal identification threshold of a receiver in the OLT and to extract the clock and a delimiter that identifies a boundary of a received signal; and a PON control signal (sometimes called an overhead or a header), preceding data (sometimes called a payload). The data is variable-length burst data and begins with a GPON encapsulation method (GEM) header, which is used to process the variable-length data.
The signal sent from the OLT to each ONU includes time-division-multiplexed data addressed to the ONU, preceded by a frame synchronization pattern for identifying the beginning, a PLOAM field for sending monitoring, maintenance, and control information, an overhead (sometimes called a header), or a grant indication field for indicating the timing of signal transmission to the ONU, so that each ONU can identify and process the signal from the OLT. Like the signal from the ONU, the multiplexed data addressed to the ONU includes a GEM header for variable-length data processing. The OLT determines upstream transmission timings (start timing (Start) and end timing (Stop)) of each ONU in bytes and gives the information to the ONU by using the grant indication field. The given transmission timing is called a grant. When each ONU sends data addressed to the OLT at the given permitted timing, the data is optically (time division) multiplexed on the optical fiber and is received by the OLT.